As a manufacturing technique, injection molding plays a significant role. Injection molding is the most important plastics processing method. Over 60% of all thermoplastics are injection molded. Injection molding is also used to produce parts from metals such as magnesium, lead, tin, zinc, aluminum and various alloys of these such as brass and other metals. The injection molding of metals is usually referred to as die casting, and may employ any of various apparatus including, for example, hot-chamber die-casting machines and cold-chamber casting machines. Injection molding may also be accomplished under vacuum.
Injection molding, whether the molding material is molten metal or plastic, involves forcing molten material under high pressure into molds called dies. The term "injection molding" is intended herein to refer to any molding or casting method which involves forcing molten material under high pressures, typically in excess of 35 kg/cm.sup.2, into a mold or die. Injecting molding is closely related to permanent mold casting in that both processes employ reusable molds. The two processes differ, however, in the mold-filling methods. Whereas mold filling in permanent mold casting usually depends on the force of gravity or low pressure, typically up to about 10 kg/cm.sup.2, injection molding involves flow into the mold cavity of hot molten material at velocities induced by the application of pressure. Because of this high-pressure filling, injection molding can produce shapes that are more complex, have thinner cross sections, higher strength and better surface finish than shapes that can be produced by permanent mold casting.
In injection molding, after the mold has been closed and locked, molten material is delivered to a pump, which may be a separate chamber or may be immersed in the molten material. The pump plunger is advanced to drive the molten material quickly through the feeding system while the air in the mold cavity escapes through vents. Sufficient molten material is introduced to fill the mold cavities and the over-flow wells. Pressure is maintained through a specified dwell time to allow the metal or plastic part to solidify. The mold is opened and the part is then ejected.
The injection molding mold usually consists of two sections which meet at the mold parting line. The two sections are typically closed and locked or clamped together to thereby define a cavity which is the counterpart of the item to be molded. Sufficient mechanical force is maintained between the mold parts to resist parting under the pressures employed in injection molding.
The mold cavity is typically machined into the two halves of the mold block or machined mold inserts are inserted into each mold block. Molds are customarily fitted with various items to assist in the casting operation, such as cooling channels, ejector pins and the like. A mold may have a single cavity or multiple cavities to form more than one part at the same time.
It is well recognized in the injection molding art that the utmost care must be exercised in the selection of the type of material used in making the molds. The most common material selected is tool steel. The composition of the materials from which the mold is formed has varied, depending upon the particular material being molded, its size and its design. Oil-hardened tool steel has been employed to produce molds of close tolerances and complicated shapes. Cheaper molds, where tolerances are not as critical, have been made of case-hardened ingot iron or machine steel. Such a mold, however, would be incapable of withstanding extremely high pressures and might tend to swell or distort over a long period of use. Steel alloys including nickel, magnesium, chromium and carbon have provided relatively satisfactory general-purpose molds.
Notwithstanding the composition of prior art molds, injection molding is extremely hard on molds. Although some last longer than others, they all suffer eventually because of distortion, erosion of the mold cavity surface, heat checking from thermal shock, and for other reasons. Various attempts have been made to improve the performance and/or life of molds by utilizing mold inserts, i.e., facings on the mold cavity of various materials which have improved properties to reduce wear, maintain tolerances, or for other purposes. For example, U.S. Pat. No. 3,709,459 (Bushrod) discloses the use of silicon nitride mold facings for use in permanent mold casting of lead or lead alloy electrode grids for electric storage batteries. Additionally, Blair et al (U.S. Pat. No. 4,139,677) discloses using a mold having molding surfaces formed of silicon carbide or nitride in a permanent mold for making glass elements.
The use of ceramic material as a facing or insert for injection molding dies is virtually unknown. This is not surprising since ceramic materials are generally known to have very poor resistance to mechanical impact as compared to the metals customarily employed to make molds. Moreover, even if one were to utilize a ceramic facing for a mold, as disclosed in either the Bushrod patent or in the Blair et al patent mentioned above, a suitable injection molding mold could not be produced by following the teachings of the prior art because ceramic materials have such a vastly different coefficient of expansion on heating than metals. The ceramic facing would expand at a different rate than the metallic backing or body of the mold and result in a misfit. Furthermore, upon injection of the molten material, the improperly fitted ceramic insert would crack or shatter because of its negligible resistance to impact and because of a lack of mechanical support due to the misfit. Additionally, while it is possible to coat a simple mold cavity wall with a facing of ceramic material, it would be difficult, if not impossible to coat a complex mold cavity wall with a uniform facing of ceramic material.